phranque - 7:24 am on Oct 5, 2010 (gmt 0)
you have to figure out where the vulnerability is to ultimately solve the problem.
for example, it could be that someone obtained your login credentials, or that another account on the shared server provided a back door, or that you installed a theme containing some "bad stuff", or that WP or a plugin has a vulnerability.
here are a few WebmasterWorld posts that may provide some clues.
Website HACKED - help!:
I have a site hijacking my wordpress - can you help?:
30 day ban for cloaked outgoing links due to PHP hack: